Crystals of 1-methylcytosine HBr are orthorhombie with a = 12-98, b=6-80, and c=8.83 A. The space group is Pnma with 4 molecules in the unit cell, all the atoms lying in the mirror phmes at y =1. an(i 3. Bond lengths and angles have been determined from the (010) projection, a(x, z)= 0.02 A, and the dimensions of the pyrimidine ring are in fair agreement with previous predictions. The acidic proton has been identified as being attached to N(3) of the cytosine ring which exists in the keto-amino form in this crystal. No inter-pyrimidine hydrogen bonds are formed, the molecules being heM together in layers, in an interlocking double dovetail arrangement, by a planar system of hydrogen bonds of the type :N-H • • • Br. The molecular packing is somewhat unusual, the Brions of one layer being sandwiched between the pyrimidine rings of the layers immediately above and below.
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Bryan et al. (1962) studied this question.